This invention relates to solid-state recrystallization (SSR) methods of growing mercury cadmium telluride (MCT) crystals suitable for semiconductor applications.
Mercury cadmum telluride (MCT or HgCdTe) is a so-called pseudo-binary semiconducting material, meaning that it is a mixture of mercury telluride and cadmium telluride which behaves as if it were composed entirely of a single metal-telluride compound. MCT is frequently used as a photo-sensing material, and the photo-energy it absorbs is very sensitive to its stoichiometry--i.e. the ratio of mercury telluride to cadmium telluride. Accordingly, it is critical when growing MCT crystals to be able to control stoichiometry.
MCT crystal growth is particularly difficult because the melting points of mercury telluride and cadmium telluride are relatively widely separated, and slow cooling of a molten mixture of the two compounds results in their segregation, with the higher melting compound solidifying first. Thus, traditional techniques which involve crystal growth by slowly cooling a melt yield crystals with unsatisfactory compositional variation. The relatively high vapor pressure of one of the components (mercury) also contributes to the difficulty experienced in controlling MCT composition, because the mercury tends to vaporize to a greater extent and at a faster rate than the other two components.
An example of a melt-growth method for making MCT ingots involves the use of a Bridgman furnace as disclosed in P. W. Kruse, Appl. Optics, Vol 4, No. 6, p. 687, 1965, and B. E. Bartlett, et. al., J. Materials Science, Vol 4, p. 266, 1969. The slow solidification in the Bridgman furnace results in a compositional gradient from one end of the ingot to the other due to segregation of the components. A relatively faster Bridgman soldification process reduces the compositional variation in the axial direction (i.e., along the axis of the cylindical ingot), but only at the expense of worsening the compositional variation in the radial direction.
To avoid the above problems, a melt of known composition may be quenched at a rate that is too fast to produce satisfactory crystallinity; crystal structure of the resulting solid may be improved by heating it to a high temperature that is still below the melting point. The high temperature provides sufficient energy to support crystal growth within small distances, which significantly improves the crystal structure. The principle of this technique, called solid phase growth or solid state recrystallization (SSR), is that rapid solidification locks in macroscopic compositional homogeneity and subsequent annealing at a temperature just below the solidus improves both microscopic compositional homogenization via solid state diffusion and grain growth via solid state recrystallization. The objective of solid phase growth is to eliminate the problem inherent in all melt-growth methods, namely, segregation due to remelt.
With these fast cooling techniques, excessive cooling rates can damage the material due to rapid radial contraction of material upon solidification which produces pitting, piping, blow holes, and cavities, or mercury vapor eruption due to cooling of the vapor space over the material. Slowing the cooling rate, however, results in detrimental loss of compositional uniformity from coring. Reducing the diameter of the ampoule ameliorates these problems to some extent but reduces the chances of obtaining single crystals large enough to fabricate the multi-element arrays now required for many applications.
Several variations of the quenching method of solid phase growth have been suggested. The ampoule may be removed from the rocking furnace, in which the raw materials have been melted and rocked for mixing, and immediately dropped into a water or ice bath. Alternately, the ampoule may be contained in a pipe in the rocking furnace and a cold gas stream directed through the pipe to quench the melt. These two methods are discussed in U.S. Pat. No. 3,468,363. In U.S. Pat. No. 3,468,363 the portion of the ampoule containing the liquid is removed from the furnace and immersed in an oil bath while the portion of the ampoule containing the vapor space is positioned inside the furnace so that the vapor space is kept hot while the liquid is being frozen.
For some years, New England Research Center and others have used a solid state recrystallization technique in which a sealed ampoule containing the three elemental materials is mounted in a pressure vessel within the rocking furnace. After the materials have been mixed and reacted, they are rapidly soldified by gas flow from an inlet at one end of the pressure vessel, along the cylindrical wall of the ampoule, and through an outlet at the other end of the pressure vessel. During at least the initial stages of this quench flow, positive pressure is maintained within the vessel at a level sufficiently high to curtail explosions. Experience at NERC has shown that the uniformity and other characteristics of crystals produced using the system are erratic.